1. Field of the Invention
The present invention relates to a technology for transmitting frames (packets). More particularly, the present invention relates to a technology that allows change in link aggregation structure without interruption to the service or loss or duplication of the frames.
2. Description of the Related Art
Ethernet (Registered trademark) has been extensively employed in Local Area Networks (LAN). However, because networks can be establishment at relatively low cost by employing the Ethernet, the Ethernet is now a days adopted in Wide Area Networks as well. For example, Ethernet is now employed as the backbone of a communication carrier.
With that perspective, there is a great demand for increasing communication speed and availability of the Ethernet. Link aggregation, stipulated by Institute of Electrical and Electronics Engineers (IEEE) 802.3ad (presently integrated with IEEE 802.3), is one such attempt for enhancing the communication speed and the availability of the Ethernet.
The link aggregation involves combining a plurality of physical data channels into a single logical link. For example, by employing the link aggregation, five 1-Gbps data channels can be combined to form one 5-Gbps link. When data channels are combined in this manner, it becomes possible to increases the bandwidth and the availability of data channel, in addition, even if a fault occurs in one of the links, communication can be continued over other normally operating links. A conventional technology has been disclosed in Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications, [online], March 2002, retrieved from the internet [URL: http://standards.ieee.org/getieee802/download/802.3-2002.pdf].
However, care needs to be taken when employing the link aggregation. That is, it is necessary to make sure that frame duplication does not occur when flooding all the links with Media Access Control (MAC) frame while broadcasting or multicasting. Frame duplication otherwise occurs in the destination device of the link aggregation. Frame duplication can be prevented by outputting frames only to one link among the links that are combined to form a link aggregate.
Conventional transmission apparatuses, such as layer 2 switches (hereinafter, “L2 switch”), use a table containing information pertaining to which ports to deliver frames to during flooding, thus, taking care that frame duplication does not occur.
However, frame duplication and/or frame loss can occur while the table is being updated. Frame duplication and/or frame loss is a serious problem when it comes to communication carriers.